Rainbow Electronics MAX5093 - Manual (Page 18)

MAX5092/MAX5093
4V to 72V Input LDOs with Boost Preregulator
18 ______________________________________________________________________________________
Design Guidelines
Input Capacitor (C IN ) and
Boost Capacitor (C
BSOUT
) Selection
The input current waveform of the boost converter is
continuous, and usually does not demand high capaci-
tance at its input. However, the MAX5092_/MAX5093_
boost converter is designed to fully turn on as soon as
the input drops below a certain voltage in order to ride
out cold-crank droops. This operation demands low
input source impedance for proper operation. If the
source (battery) is located far from the IC, high-capaci-
ty, low-ESR capacitors are recommended for C
IN . The
worst-case peak capacitor current could be as high as
3A. Use a 47µF, 100mΩ low-ESR capacitor placed as
close as possible to the input of the device. Note that
the aluminum electrolytic capacitor ESR increases sig-
nificantly at cold temperatures. In the cold temperature
case, choose an electrolyte capacitor with ESR lower
than 40mΩ or connect a low-ESR ceramic capacitor
(10µF) in parallel with the electrolytic capacitor.
The boost converter output (BSOUT) is fed to the input
of the internal 250mA LDO. The boost-converter output
current waveform is discontinuous and requires high-
capacity, low-ESR capacitors at BSOUT to ensure low
V
BSOUT ripple. During the on-time of the internal MOSFET,
the BSOUT capacitor supplies 250mA current to the
LDO input. During the off-time, the inductor dumps cur-
rent into the output capacitor while supplying the output
load current. The internal 250mA LDO is designed with
high PSRR; however, high-frequency spikes may not be
rejected by the LDO. Thus, high-value, low-ESR elec-
trolytic capacitors are recommended for C
BSOUT .
Peak-to-peak V BSOUT ripple depends on the ESR of the
electrolyte capacitor. Use the following equation to cal-
culate the required ESR (ESR
BSOUT
) of the BSOUT
capacitor:
where ∆V ESRBS is 75% of total peak-to-peak ripple at
BSOUT, I LIM is the internal switch current limit (3A max),
and I OUT is the LDO output current. Use a 100mΩ or
lower ESR electrolytic capacitor. Make sure the ESR at
cold temperatures does not cause excessive ripple
voltage. Alternately, use a 10µF ceramic capacitor in
parallel with the electrolyte capacitor.
During the switch on-time, the BSOUT capacitor dis-
charges while supplying I
OUT . The ripple caused by
the capacitor discharge (∆V
CBS
) is estimated by using
the following equation:
where I OUT is the LDO output current and C BSOUT is
the BSOUT capacitance.
Inductor Selection
The control scheme of the MAX5092/MAX5093 permits
flexibility in choosing an inductor value. Smaller induc-
tance values typically offer smaller physical size for a
given series resistance, allowing the smallest overall
circuit dimensions. Circuits using larger inductance
may provide higher efficiency and exhibit less ripple,
but also may reduce the maximum output current. This
occurs when the inductance is sufficiently large to pre-
vent the LX current limit (I
LIM ) from being reached
before the maximum on-time (t ON-MAX ) expires.
For maximum output current, choose the inductor value
so that the controller reaches the current limit before
the maximum on-time is reached:
where t ON-MAX is typically 2.25µs, and the current limit
(I LIM ) is a maximum of 3A (see the Electrical
Characteristics ). Choose an inductor with the maximum
saturation current (I
SAT
) greater than 3A.
L
V t
I
IN ON MAX
LIM
≤
×
−
∆V
I
C
CBS
OUT
BSOUT
=
Ч Ч
−
2 7 10
6
.
ESR
V
I I
BSOUT
ESRBS
LIM OUT
=
−
∆